Take the handle off and you will probably find that the shaft has come adrift - the handle turns but the shaft doesn't. Clean it up and push the shaft back into the handle and try it before refitting - maybe use something a bit sticky to stop it happening again.
Had a look at taking the lock off, but there are no visible screws or fixings so it seems that I will have to remove the bottom of the window to get in and have a look. Stupid Everest windows that I've inherited with the house.
